The overall goal of these experiments is to reveal a mechanistic basis for the inverse correlation between smoking and Parkinson's disease. The project tests whether chronic exposure to nicotine up-regulates functional nicotinic acetylcholine receptors (nAChRs) in the basal ganglia, changing cell and circuit function. The cell-specific alpha4 up- regulation (CSAUR) hypothesis states that the key up-regulated receptors are characterized by a high sensitivity (HS) to nicotine and are therefore mostly of the alpha4beta2* and possibly of the alpha6* subtypes; that cell-specific increases in alpha4* nAChR(s) occur during chronic exposure to nicotine; and that this cell-specific upregulation provides the basis for circuit-based changes in neuronal activity. Aim 1 uses mouse brain slices containing basal ganglia to detect the distribution of alpha4* receptors, then to determine CSAUR. The studies will employ, where appropriate, quantitative electrophysiological assessment of nAChR function, mouse strains with modified nAChRs, cell labeling, and immunohistochemical techniques. Substantia nigra pars compacta (SNc) will be studied to confirm the absence of up-regulation in these neurons. The hypothesis will be tested that chronic nicotine renders SNc neurons less sensitive to burst firing mediated by GABAA receptor blockade or by NMDA receptor activation. Subthalamic nucleus (STN) will be studied to confirm preliminary data suggesting that it expresses a* receptors. If CSAUR occurs in STN, burst firing will be studied. Medium spiny neurons (MSNs) in dorsal thalamus will be studied to determine whether chronic nicotine changes alpha4* function at the DA terminals. If so, electrochemical detection of DA release will be used. GABAergic interneurons (INs) will be tested for alpha4* receptors, and then for alpha4* up-regulation. If CSAUR is detected, the alpha4*-expressing INs will be identified by anatomy; and the particular subclass of inhibited MSNs will be identified. In Aim 2, single DA and GABAergic cells will be monitored in basal ganglia of intact animals, to assess effects of chronic nicotine that are revealed when neuronal circuits are intact . In Aim3, fluorescence will be measured in knock-in mice that express fully functional fluorescent alpha4* receptors. Up-regulation of this fluorescence will be measured in specific cell types. High-resolution subcellular studies with 2-photon microscopy will determine the sub-cellular specificity of alpha4* expression and up-regulation. Understanding the cellular and circuit changes induced by chronic nicotine in basal ganglia could lead to new therapies for Parkinson's disease.